This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The abbreviations and terms appearing in the description and drawings are defined as below.
3GPP Third Generation Partnership Project
ABS Almost Blank Subframe
BS Base Station
CBS Complete Blank Subframe
CDMA Code Division Multiple Address
CRS Common Reference Signal
DL Downlink
DMRS Demodulation Reference Signal
eICIC Enhanced inter-cell interference coordination
eNB enhanced NodeB, Base Station in E-UTRAN
E-UTRAN Evolved UTRAN
FDD Frequency Division Duplex
HO HandOver
IE Information Element
LTE Long Term Evolution
LTE-A Long Term Evolution-Advanced
PCI Physical Cell Identifier
PDCCH Physical Downlink Control Channel
PDSCH Physical Downlink Shared Channel
PHICH Physical HARQ Indicator Channel
PSS Primary Synchronization Signal
RRC Radio Resource Control
RS Reference Signal
RSRP Reference Symbol received power
SON Self-optimization network
SSS Secondary Synchronization Signal
TDD Time Division Duplex
UE User Equipment
UL Uplink
UTRAN Universal Terrestrial Radio Access Network
According to LTE specification, each cell in an LTE network is configured with a physical cell identifier (PCI), which determines the physical channel configuration in this cell, including downlink and uplink physical control channels and data channels, as well as a variety of reference signals and synchronization signals. Therefore, in LTE network deployment and network optimization, some rules are given to configure the PCI values for a group of neighbour cells. One important rule is to avoid “PCI collision” in which two neighbour cells have the same PCI values.
However, in practice, quite many factors could lead to the occurrence of a PCI collision, such as the fault in network dimensioning, neighbour cell relation change due to the radio propagation environment variation, the side effect or fault in PCI optimization (a kind of regular self-optimization network (SON) operation), and the employment of heterogeneous network (like pico-cell, femto, relay cell) etc. Once a PCI collision is spotted, the PCI reconfiguration for at least one of the relevant cells should be executed. Because the PCI determines the basic physical layer parameters, its reconfiguration usually needs to shut down the cell and then assign another suitable PCI value off-line. During this procedure, user equipments (UEs) served by this cell should be moved outward to neighbour cells as much as possible. The approach is to set a certain time period in which the UEs served by this cell are assisted to move outward, by either handover or release with redirection. This procedure is called as “cell soft-lock”, which belongs to one function of self-optimization network (SON), as shown in FIG. 1.
FIG. 1 shows the process of cell soft-lock upon PCI collision. As shown in FIG. 1, at the time 110, a PCI collision is spotted. Then, a time window 120 is set for the UEs to move out of the cell. During this process, connection and service dropping should be minimized. After this, at the time 130, the cell is shut down. Then, the PCI reconfiguration may be executed during the period 140.
To guarantee the user experience, minimizing the dropping rate is one important object in the cell soft-lock procedure. However, for those UEs which lie in the border of two cells with identical PCI value (i.e., PCI collision), it becomes very difficult for them to take handover from one cell to another, because the identical PCI makes the physical layer control channels as well as the data channels in these two cells collide with each other.